1. Field of the Invention
The present invention relates to a cathode-ray tube apparatus.
2. Description of the Related Art
In order to correct an edge of a displayed image to enhance image quality, a method for modulating the horizontal scanning velocity of electron beams is known. According to this method, in general, a pair of auxiliary coils called beam velocity modulation (BVM) coils are provided at a neck portion of a cathode-ray tube so as to be integrated with a deflection yoke and a convergence and purity unit (CPU) (see JP57(1982)-45650Y).
The BVM coils improve a visible state between a dark area and a light area of an image displayed on a screen during a horizontal scanning period as follows. The transition of a lighting state is predicted from a video signal waveform. In a period on a dark side during a transition period of a lighting state, electron beams are accelerated so that they are horizontally scanned at a velocity equal to or higher than an average scanning velocity. On the other hand, in a period on a light side during the transition period of a lighting state, electron beams are decelerated so that they are horizontally scanned at a velocity equal to or lower than the average scanning velocity. Thus, among the areas of the transition of a lighting state on a screen, in an area on a dark side, an excitation time of phosphors is shortened to decrease brightness, and in an area on a light side, an excitation time of phosphors is prolonged to increase brightness. Accordingly, the edge of an image is corrected so as to increase the sharpness of the areas of the transition of lightness and darkness.
In the case where the above-mentioned BVM coils for modulating the horizontal scanning velocity of electron beams are provided so as to be integrated with the CPU, an eddy current is excited in an electron gun made of a metallic conductor due to the magnetic flux generated by the BVM coils, and the metallic conductor generates heat, resulting in a reduction in the velocity modulation effect by the BVM coils.
In order to enhance the sensitivity of the BVM coils, a method has been proposed for mounting a magnetic substance, which focuses and intensifies the magnetic field generated by the BVM coils, in the electron gun (see JP6(1994)-283113A).
However, the above-mentioned method has the following problems: since a magnetic substance for focusing a magnetic field is metal, a new eddy current is likely to be generated; a process for welding a new component is necessary, which increases a cost; and furthermore, sufficient sensitivity cannot be obtained.
Furthermore, it also is considered that, in order to decrease crosstalk between the magnetic field generated by the BVM coils and the electron gun metal to minimize the generation of an eddy current, the BVM coils should be placed close to the end of a deflection yoke on the electron gun side. However, in this case, the BVM coils are placed close to a horizontal deflection coil, so that crosstalk between the magnetic field generated by the BVM coils and the horizontal deflection magnetic field is increased to cause new ringing. Therefore, there is a limit for placing the BVM coils close to the deflection yoke, and hence, a sufficient increase in sensitivity cannot be realized.